MTS Project 3: Environmental Durability of Adhesive Bonds. Report No 11. Locus of Failure of Bonded Joints

Transcription

1 MTS Project 3: Environmental Durability of Adhesive Bonds Report No 11 Locus of Failure of Bonded Joints DM Brewis, GW Critchlow, I Mathieson K Ebtehaj December 1996

2 Customer Ref: AH 9/3 Document Ref: AEAT-229 File No: 2957/51 Document Series: REPORT Title: Report 11 Locus of Failure of Bonded Joints ISSUE RECORD Issue Date Author Checked By Approved By 1 December 1996 D M Brewis G W Critchlow I Mathieson K Ebtehaj G C Eckold G C Eckold

3 Foreword Many UK manufacturers are aware of the merits of adhesives in certain critical roles. However, the range of applications of adhesives is still limited largely due to the lack of consistent test methods and validated test data which the engineer needs in order to specify adhesives for a given application. In a recent survey the Centre for Adhesive Technology was commissioned by the DTI to establish specific areas where validated test methods could improve confidence in predicting joint life. The survey identified measurement methods for use in design, environmental durability and process control as priority areas and five projects were finally selected by the DTI for support through the Measurement Technology and Standards (MTS) budget. The projects started in December 1992 and are 1% funded by the DTI at the level of 5.4M over three years. A key area that was identified in the survey was the need to provide a means for the quantification and prediction of the lifetime of bonded joints when exposed to a hostile environment, and this forms the basis of Project 3 of the Adhesives programme. The project is being carried out through a collaboration of AEA Technology and Oxford Brookes University, together with important contributions from Imperial College, DRA (Holton Heath) and Loughborough University. The basis of the project is that there still remains considerable uncertainty in predicting joint lifetime under typical operating conditions, despite the enormous prior investment in durability studies. This difficulty represents perhaps the single most significant impediment to the wider use of adhesive technology. The programme combines a fresh look at testing procedures and predictive methods with a rigorous examination of previous work. Together the two strands of the approach produce a work plan which has a strong industrial focus. In addition to tasks on test methods and design, a series of forensic studies will be undertaken on examples of bonded structures which have seen extensive service life, in order to evaluate the reasons for their success. This will provide practical feedback into the other aspects of the programme as well as indicating to designers tangible evidence of the potential of bonding technology which they will be able to relate to their own particular application. This project is divided into seven tasks: Appraisal of Test Methods and Durability Data Development of an Experimental Database Characterisation of the Moisture Absorption Process Forensic Studies Assessment of Microstructural Failure Mechanisms Development of Methodologies for Life Prediction Technology Transfer AEA Technology i

4 SUMMARY This report describes a range of surface analysis work aimed at the determination of the locus of failure in bonded joints of various geometry and material types. The work was carried out as part of Task 5: Microstructural Failure Mechanisms and used samples arising from Task 2: Experimental Database. Detailed observations were made using a variety of techniques including optical magnification, contact angle, -ray photoelectron spectroscopy and Auger electron spectroscopy. In broad terms for joints where a poor level of surface treatment was applied prior to bonding the superficial visual observation of interfacial failure was confirmed, whereas with the better methods of preparation there was significant evidence of mixed interfacial and cohesive modes. AEA Technology ii

5 Contents 1 INTRODUCTION EPERIMENTAL MATERIALS OBSERVATION WITH MAGNIFYING GLASS AND MICROSCOPE USE OF WATER CONTACT ANGLES RAY PHOTOELECTRON SPECTROSCOPY (PS) AUGER ELECTRON SPECTROSCOPY (AES) RESULTS OBSERVATION WITH A MAGNIFYING GLASS AND A MICROSCOPE USE OF CONTACT ANGLES RAY PHOTOELECTRON SPECTROSCOPY (PS) AES DISCUSSION CONCLUSIONS...6 AEA Technology iii

6 1 Introduction Under Task 2 of Project 3 several different tests were used to assess the durability of adhesive Joints; this work is described in Report No. 8 entitled Experimental Assessment of Durability Test Methods. The main joint configuration used was a single overlap, but T-peel, wedge and tensile butt tests were used. Several more novel tests were also examined. These involved stressed perforated lap joints, a fabric peel method, a bead flexure test and a polymer film test. In addition, an analytical test involving tapered double cantilever beams was also studied. In most cases in the programme "apparent interfacial" failure was the predominant mode of failure as determined by the naked eye or with a magnifying glass. However, it was recognised that the actual locus of failure could be one of several possibilities. In the case of a bonded metal joint it could be failure within the metal oxide, true interfacial failure between the metal oxide and adhesive, or cohesive failure within the adhesive or a combination thereof. In the cases of the primed and chromic acid treated polypropylene (PP), there are other possible zones of failure. In the more complicated case, i.e. the primed PP, failure could be within the substrate, at the PP-primed interface or cohesively within the primer or a combination of these. This report describes a reappraisal of locus of failure for joints used in the durability tests for Task 2. All failed joints were examined with a magnifying glass and also with a microscope with up to x45 magnification. In selected cases SEM was used. Preliminary studies using Auger electron spectroscopy were very informative and it was decided to concentrate on this technique to ascertain the true loci of failure of bonded metal joints. For the PP joints it was decided to use water contact angles and -ray photoelectron spectroscopy (PS). 2 Experimental 2.1 Materials The substrates used were mild steel (CR1), aluminium (alloy 5251) and polypropylene (Propathene GWM 213); in some cases the mild steel was primed with a silane (Union Carbide A187). The chlorinated polypropylene primer Hardlen 14-LWP was applied from a 1% solution by weight in toluene. Four adhesives were used namely Araldite 21 (a 2-part epoxide), Araldite 27 (AV part epoxide), 3Ms 3532 (2-part polyurethane) and Flexon 241 (catalysed acrylic). 2.2 Observation With Magnifying Glass And Microscope Two corresponding failed surfaces were first examined side-by-side with a magnifying glass. Individual substrates were then examined with a Gallenkamp microscope providing up to x45 magnification. AEA Technology 1

7 2.3 Use Of Water Contact Angles Advancing and receding water contact angles were first determined, using a Krüss G4 goniometer, for the following surfaces: untreated Propathene GWM 213, Hardlen 14-LWP treated GWM 213 and the cured epoxide adhesive. Failure surfaces from T-peel tests were also examined Ray Photoelectron Spectroscopy (PS) Failure surfaces of Hardlen primed PP joints and mild steel were examined with PS; corresponding faces of failed joints were examined on both unaged controls and joints immersed in water at 6 C for periods of up to 1 hours. In addition, PS analyses were carried out on the adhesive, Araldite 21, and on the primer Hardlen 14-LWP PS experiments were carried out using a VG ESCALAB Mk 1 instrument using Al k a - rays of energy ev and a power of 2W. Measurements were made using fixed analyser transmission and with the analyser normal to the plane of the sample at a pass energy of 1 ev. All peaks were charge-referenced to the major C-C/C-H 1s peak at ev. Surface compositions were calculated using the areas of the respective photoelectron peaks after subtraction of a Shirley-type background. Corrections were made for angular asymmetry of photoemission, transmission of the energy analyser, photoionisation crosssection and the inelastic mean free path. 2.5 Auger Electron Spectroscopy (AES) Surface, and in most instances sub-surface, compositions were determined on selected aluminium alloy and mild steel joints which had previously been disbonded during durability tests. In all cases the selected joints exhibited "apparent interfacial" failure when viewed optically. The aluminium systems which were analysed are shown in Table 1. Auger electron spectroscopy (AES) was combined with sequential argon ion bombardment to provide in-depth information. AES was carried out using a Varian AES spectrometer operating at a base pressure of ~5x1-9 torr. Analytical conditions were varied depending upon the sample under investigation. To reduce the influence of sample charging when analysing rough, insulating surfaces (such as areas of exposed adhesive) a reduced primary beam energy and current were used. The analytical conditions were as follows; primary electron beam energy 1 or 3keV, current.3 or.45ma, scan range 2 to 6eV or 2 to 14eV. In all cases an analysis area of ~15 mm diameter was chosen. For depth profiling AES was combined with sequential argon-ion bombardment. The ion beam energy was 3keV and the current density was 5mA.cm -2. Quantification was achieved using a combination of theoretically and experimentally-derived relative sensitive factors (RSFs) with Al:O ratios in particular based upon an Al 2 O 3 reference material. Depth scale calibration was achieved using both experimentally and theoretically derived etch rates with a theoretical value used for the organic material and an empirical etch rate for the oxide based upon a porous (Al 2 O 3 ) chromate-phosphate layer on Al. It should be noted that when using the reduced beam energy the sensitivity of this technique to oxygen is much reduced; in this case the detection limit for oxygen is ~5-6%. In all cases corresponding areas of failed aluminium joints were examined and these are designated areas, Y and Z. AEA Technology 2

8 3 Results 3.1 Observation With A Magnifying Glass And A Microscope The results of this work are summarised in Tables 2 and Use Of Contact Angles The contact angles for failed surfaces and also for the Hardlen primer and the epoxide adhesive 21 are given in Table Ray Photoelectron Spectroscopy (PS) Elemental analysis for both sides of failed primed PP joints and also for the Hardlen primer and the epoxide adhesive 21 are given in Table 5. Data for mild steel joints are given in Table AES Sample CAE/27/19 days immersion in 6 C DI water This joint had apparent interfacial failure over the entire bonded area. In approximately five percent of the surface there was adhesive attached to the metal side with a corresponding hole in the adhesive. Analysis was carried out on the top surface of the residual adhesive visible on the metal side (areas and Y), and, in the corresponding hole on the adhesive side (area ). The presence of high levels of C, along with N, on both surfaces and the absence of Al on either indicates cohesive failure within the epoxide adhesive in this region. The Si observed is possibly from exposed ballotini spheres used in the assembly of the joints. The results are given in Table 7. No adhesive could be visually observed in area Z, a representative area on the metal side. Only a limited amount of C (~16%), which is equivalent to about half a monolayer, is present in this area, with Al present at the surface. No C could be detected at a depth of 6nm. High levels of C along with Cl, Ca and N are present in the corresponding area (Z) on the adhesive side and no Al indicates true interfacial failure rather than failure in the metal oxide. The results from this sample indicate that although there are areas of cohesive failure within the joint the majority of the failure was interfacial. There is sub-monolayer coverage of organic material residual on the exposed metal surface after disbonding. Sample CAE/27/83 days immersion in 6 C DI water The joint exhibited apparent interfacial failure over most of the bonded area. There were, however, two strips along the long axis of the joint ~2mm from the edges and ~2mm wide where localised cohesive failure of the adhesive had occurred. AEA Technology 3

9 Analyses were carried out close both to the centre (area ) and the edge (area Y) on both metal and adhesive sides after testing. The results are presented in Table 8. Similar results were obtained in both the above-mentioned areas with O, Al, C, Ca and N present on the metal surface and O, Cl, C and Si on the adhesive side. The continuing presence of C (~16%) with depth on the metal surface indicates that the adhesive is not in a continuous film but is in discrete patches indicating areas (albeit resolved using the SEM facility on the AES spectrometer) of cohesive failure within the adhesive. There was no Al on the adhesive side of the failed joint indicating that, in this example, there is largely interfacial failure with localised areas of cohesive failure within the adhesive. Sample CAE/3532/19 days immersion in 6 C DI water The majority of the exposed metal surface on this joint was either matt or shiny metallic grey in appearance. Some areas close to the edges of the joint appeared to be matt brown in colour. The corresponding adhesive side of the failed joint appeared uniformly (pale yellow) coloured all over. Results (see Table 9) from a matt brown discoloured area close to the edge of the metal surface (area ) indicates that there is a thick aluminium oxide (Al 2 O 3 ) present in this region extending to >36nm. The majority of the metal surface appeared either shiny (eg. area Y) or matt (eg. area Z) grey coloured. Results from the shiny grey area indicate the presence of O, Al, S, Cl, C, Ca, Si and Cr at the surface. The S, Cl, Si and Cr are surface specific, however, the C and Ca are present throughout the oxide. The oxide is ~ 25nm thick in this area. In the matt grey area the oxide is mainly stoichiometric Al 2 O 3 but is much thicker (>18nm) than in the shiny area. Analyses carried out in two general areas ( and Y) on the adhesive side indicate that there is little or no adhesive on the immediate surface this being predominantly aluminium oxide. These results indicate cohesive failure within the metal oxide. Sample PAA/27/control (no immersion) Joint failure was apparently interfacial across the entire bonded area with no adhesive visible on the metal surface. Similar elements (O, C and N) were observed on both metal and adhesive sides indicating cohesive failure within the adhesive; no aluminium was present. The results presented in Table 1 are representative of those obtained from a number of areas on both surfaces. Sample PAA/21/2 days immersion in DI water 6 C Apparent interfacial failure was visible across the entire bonded area. SEM carried out on the metal side of the failed joint showed the honeycomb structure characteristic of the PAA oxide. There was no evidence for the presence of adhesive on this surface. Similar surface analysis results were obtained to those from the PAA/27 control sample (see Table 11). However, in this case profiling was carried out to a depth of ~1nm on both metal and adhesive surfaces to investigate whether near-interfacial failure occurred. The results show only C and N present (characteristic of the adhesive) throughout the analysis AEA Technology 4

10 depth, thus indicating cohesive failure within the adhesive; the absence of O is likely to be due to poor sensitivity to this element under the analytical conditions used. Sample Degrease/21/immersion in DI water at 45 C stressed to 2N Apparent interfacial failure was visible across the entire bonded area, as indicated both optically and by SEM. Analysis was carried out in a region close to the perforation on the metal side and in the corresponding area on the counterface. Results from the metal side show O, Al, S, Cl, C and Ca present on the surface. The C is observed mainly at the surface indicating that there is approximately monolayer coverage rather than particles present. The oxide is 3-35nm thick. On the adhesive side, there is C, N and O present within the top few nanometres but no Al. The results are indicative of solely adhesive present on this side. The results from both surfaces (see Table 12) indicate near-interfacial failure but possibly within the adhesive layer adjacent to the metal. The high O levels in the surface region of the adhesive suggest hydrolysis of the adhesive in this region as a consequence of immersion in water. Mild Steel Joints Apparent interfacial failure was observed for unaged and aged joints using Araldite 21 adhesive. This was the case for both grit blast and silane surfaces. On closer examination there was some evidence of adhesive of the adherend surface which was confirmed by the AES results (Table 13). For 27 the unaged joints failed cohesively albeit close to the interface. Aged joints failed similarly to 21 except a halo was apparent on the surface which became smaller with time. 4 Discussion Most failed metal joints (mild steel and aluminium alloy) exhibited either apparent interfacial failure or mixed apparent interfacial and cohesive within the adhesive with the interfacial normally being predominant. Degreased metals were particularly prone to apparent interfacial failure. In only two cases was failure completely cohesive; these occurred when PAA treated aluminium alloy L61 was bonded with either Araldite 27 (AV-119) or 3Ms It was very difficult to ascertain the locus of failure by optical means for PP bonded joints especially with those which had been primed. All the samples examined by AES had exhibited apparent interfacial failure. However, detailed AES analysis showed that a range of failure modes had, in fact, occurred. The results for aluminium are summarised in Table 14. The use of water contact angles to determine the locus of failure for the PP joints was unsuccessful. This was because roughening of the surfaces took place during the breaking of the joints and this led to large changes in the contact angles (Table 4). It was not possible, therefore, to identify failure surfaces from contact angles. AEA Technology 5

11 The use of -ray photoelectron spectroscopy to identify failure modes with the PP joints was much more successful (see Table 5). The control joints, ie. no immersion in water, showed mainly interfacial failure between the PP and the primer with some cohesive failure within the primer. The oxygen observed in one case probably originates from the primer which is believed to possess oxygen-containing species, although these were not observed in the PS analysis of the primer. The joint that had been immersed in water for 11 days exhibited cohesive failure within the PP with some interfacial failure. At 3 days failure appeared to be mainly interfacial between the PP and primer, with some cohesive failure in the PP and a small amount of cohesive failure of the primer. After 72 days, there was a significant difference in the two joints analysed, probably due to different stresses in the two areas analysed. However, in both cases cohesive failure in the PP appeared to be an important mechanism. The fact that no nitrogen was observed on any of the failed surfaces means there was no interfacial failure between the primer and adhesive nor was there any cohesive failure within the adhesive. Thus the weakest part of the joint is at or near the PP - primer interface. The increased tendency towards cohesive failure within PP may be due to migration of low molecular weight hydrocarbons to the near surface of the polymer causing a weakening. However, this is speculation. 5 Conclusions Observation of a wide range of joints using a magnifying glass or a microscope (x45) showed apparent interfacial failure to be the most common mode of failure. In some cases, especially with degreased metals, apparent interfacial was the sole mode of failure. Mixed cohesive and apparent interfacial failure was seen with the better surface treatments. In only two cases, both anodised aluminium in a T-peel test, was failure completely cohesive within the adhesive. Auger electron spectroscopy showed that apparent interfacial failure in bonded metal joints could in reality be one of three types of failure, namely, true interfacial, cohesive within the adhesive or failure within the metal oxide. The use of water contact angles could not resolve the locus of failure with primed polypropylene joints. However, -ray photoelectron spectroscopy demonstrated that failure occurred at or near the interface between polypropylene and the primer. There is an increasing tendency for cohesive failure within the polypropylene with increasing exposure. It is clear that misleading conclusions regarding the locus of failure can be made without the use of surface sensitive analytical techniques. AEA Technology 6

12 Description (treatment/adhesive) Joint type Exposure (time/condition) Failure load or Time-to-failure CAE/27 T-Peel 19 days, 6 C immersion in DI water CAE/27 T-Peel 83 days, 6 C immersion in DI water CAE/3532 T-Peel 19 days, 6 C immersion in DI water PAA/27 T-Peel Control, no immersion PAA/21 T-peel 2 days, 6 C immersion in DI water Degrease/21 Perforated SLS 45 C immersion, stressed to 2N.8kg/2mm width 3.2kg/2mm width 1.kg/2mm width 1kg/2mm width 5.1kg/2mm width 4 hours Table 1. Aluminium Specimen Details AEA Technology 7

13 System Test Observation Mild steel (GB) - 21 Single lap/butt I + some C Mild steel (A187) - 21 Single lap/butt I + some C Mild steel (DEG) - 21 Single lap/butt I Mild steel (GB) - 27 Single lap/butt I + some C Mild steel (GB) F241 Single lap I + some C Polypropylene (CAE) - 21 Single lap I (primer - 21) Polypropylene (14-LWP) 21 Single lap Mixed I (ie. PP - primer and primer - 21) Polypropylene (CAE) Single lap I (primer ) Polypropylene (14-LWP) Single lap Mixed I (as above) Aluminium (DEG) - 21 Single lap I Aluminium (GB) - 21 Single lap I + some C Aluminium (A187) - 21 Single lap I + C Aluminium (CAE) - 27 T-peel I Aluminium (CAE) T-peel I Aluminium (PAA) - 27 T-peel C Aluminium (PAA) - 21 T-peel I + some C Aluminium (PAA) T-peel C Polypropylene (CAE) - 21 T-peel I Polypropylene (14-LWP) - 21 T-peel? I - Apparent interfacial failure C - Cohesive failure in the adhesive? - Unclear Table 2. Optical observation of locus of failure for single lap and T-peel joints AEA Technology 8

14 Pretreatment Adhesive Stress/kN Locus of failure DEG 21.2 I GB 21.2 I GB I GB/A I GB/A I FPL I opt FPL I PAA 21.2 I PAA I PAA I DEG 27.2 I GB I + some C GB I + some C GB I + some C GB/A I + some C FPL I + some C FPL I opt FPL I + some C opt FPL I + some C opt FPL I + some C PAA I PAA I PAA I + some C PAA I + some C Table 3. Locus of failure by optical methods for aluminium stressed perforated lap joints AEA Technology 9

15 Sample Advancing Contact Angle Receding Araldite Polypropylene Hardlen Control joint side 1 side 2 Joint immersed side 1 for 3.1 days side 2 Joint immersed side 1 for 72 days side 2 Joint immersed side 1 for 72 days side 2 97 > > > <1 67 ~ <1 Table 4. Contact angles for failed T-peel joints of primed polypropylene bonded with Araldite 21 and for individual components AEA Technology 1

16 Period of immersion / days Side Elemental analysis by PS (atom %) Comments C Cl O N Mainly I between PP and primer with some C in primer Mainly I between PP and primer with some C in primer Mainly C within PP with some I between PP and primer Mainly I between PP and primer; some C in PP; small amount of C in primer Mainly I between PP and primer; some C in PP; small amount of C in primer C in PP and primer C in PP; I between PP and primer, small amount of C in primer C - Cohesive failure, I - Interfacial failure The elemental analysis (atom %) of the Hardlen primer and the epoxide adhesive 21 were as follows: C Cl O N Primer Epoxide Table 5. PS analysis of failed primed PP - 21 T-peel joint AEA Technology 11

17 Period of Elemental Analysis by YPS (atom %) Immersion hrs C O N Si Fe Ca Na S Al Cl Comments Araldite 27, grit blast, C near the interface Araldite 27, Silane, C near the interface Araldite 21, Silane, C near the interface Araldite 21, C near the interface Table 6. PS Analysis of failed mild steel joint AEA Technology 12

18 Depth (nm) Analysis area O Al(o) S Cl C Ca N Si Metal side 6 (adhesive patch) Y (adhesive patch) Z (general area away from adhesive) Z Adhesive side (hole in adhesive) Y (general area) Z (general area) Table 7. Composition atom % - Sample CAE/27/19 days immersion in 6 C DI water Depth (nm) Analysis area O Al(o) Al(e) Cl C Ca N Si Metal side 18 9 (close to centre) Y (close to edge) Y Y Adhesive side (close to centre) Y (close to edge) Table 8. Composition atom % - Sample CAE/27/83 days immersion in 6 C DI water AEA Technology 13

19 Depth (nm) Analysis area O Al(o) Al(e) S Cl C Ca Si Cr Metal side (corroded area) Y (shiny grey) Y Y Y Y Y Z (matt grey) Z Z Z Adhesive side Y Y Y Table 9. Composition atom % - Sample CAE/3532/19 days immersion in 6 C DI water AEA Technology 14

20 Depth (nm) O C N Metal side Adhesive side Table 1. Composition atom % - Sample PAA/27/control (no immersion) Depth (nm) Analysis area O C N Metal side Adhesive side Table 11. Composition atom % - Sample PAA/21/2 days immersion in DI water 6 C AEA Technology 15

21 Depth (nm) Analysis area O Al(o) Al(e) S Cl C Ca N Metal side Adhesive side Key to Tables Al(o) indicates Al present in a high oxidation state eg. Al 2 O 3. Al(e) indicates Al present in its zero valent (or metallic) form. Table 12. Composition atom % - Sample Degrease/21/immersion in DI water at 45 C stressed to 2N AEA Technology 16

22 Sample Depth Co O Si Fe Ca S Cl (nm) Gritblast, (unaged) Silane, (unaged) Gritblast, (aged) Silane, (aged) Table 13. Composition atom % - Mild Steel AEA Technology 17

23 Sample description CAE/27/19 days immersion in 6 C DI water CAE/27/83 days immersion in 6 C DI water CAE/3532/19 days immersion in 6 C DI water PAA/27/control (no immersion) PAA/21/2 days immersion in DI water 6 C Degrease/21/immersion in DI water at 45 C stressed to 2N Locus of failure mainly interfacial with a small amount of cohesive failure within the adhesive mainly interfacial with a small amount of cohesive failure within the adhesive cohesive failure within the metal oxide cohesive within the adhesive cohesive within the adhesive cohesive failure within the adhesive but close to the interface Table 14. Summary of AES study on locus of failure for aluminium joints AEA Technology 18